1. Field of the Invention
The present invention relates to a partial doping technology with an impurity, which is necessary for a preparing process of such semiconductor device as MOS (Metal-Oxide-Semiconductor) type or CMOS (Complementary Metal-Oxide-Semiconductor) type semiconductor device. In particular, the present invention provides a doping technology, which is capable of a selective method to dope a different region with a different impurity, using a simple and convenient process, and also which is capable of an efficient doping in a low temperature process.
2. Description of the Related Art
It is indispensable to arrange a process for partially controlling of a resistance rate, by adding an impurity which selectively gives one conductivity type to a part of semiconductor, in case where such semiconductor device as MOSFET (Metal-Oxide-Semiconductor Field-Effect-Transistor) or CMOS type device is prepared.
In a conventional process, an impurity doping has been carried out by the following method. First of all, a shield film is formed on a surface of semiconductor to keep away the intrusion of impurity. Then, the shield film in the region, where a doping will be effected in accordance with a photolithography process, is removed to form a mask. After that, the doping with a needed impurity is executed by a heat-diffusion method or an ion-implantation method.
However, such doping method in the conventional process as mentioned above creates the following problems.
(1) In case where an impurity is doped in a semiconductor by a heat-diffusion method, there poses a problem that a high temperature process is required. For example of a silicon semiconductor, it is necessary to heat a silicon semiconductor sample at a temperature of 1000 to 1200° C., thereby making it difficult to form a shallow impurity layer which is required for a high density IC, and posing a problem of impurity redistribution and defect resulted from the high temperature process.
(2) In case of the impurity doping method by an ion-implantation, there poses the same problem as mentioned in the above (1), because it is in need of a post heat-treatment at a temperature of 600 to 950° C., to activate the impurity and to recover the defect.
Also, as a problem in common with the heat-diffusion method and the ion-implantation method stated above, both of them need a high temperature process extremely over 600° C. For example, in case of an active matrix type liquid crystal display device to which an attention has been paid recently, since MOS type thin film transistor (TFT) is formed on a glass substrate, it has been difficult to employ the above heat-diffusion method and the ion-implantation method, if a cheap glass substrate which has a heat resistant temperature of 600 to 700° C. is used.
Further, in case of a selective doping, it is needed to form a mask as mentioned above. Then, a photolithography process followed by a complicated process will be required, and it has been well known that the photolithography causes a yield to be lowered.
As noted above, there has been a preparing problem that the high temperature process is required in the conventional impurity doping method, and further, a mask forming process, which is in need of a photolithography process for the selective doping, is required. So far, as a doping technology, a heat-diffusion method and an ion-implantation method have been known. In the heat-diffusion method, an impurity will be diffused into a semiconductor at such a high temperature as 1000 to 1200° C., and in the ion-implantation method, an ionized impurity will be accelerated in an electric field to be implanted in a prescribed place.
However, the diffusion coefficient of impurity, D is shown as D=D0 exp[−Ea/kT], and it is dependent on the absolute temperature T by the exponential function. Here, D0 is a diffusion coefficient at T=∞, and k is Boltzman coefficient. Then, it has been preferable to effectively diffuse the impurity into a semiconductor at as high temperature as possible, and it has been generally conducted at a temperature above 1000° C., in case of the heat-diffusion method. Also in case of the ion-implantation method, it has been needed for the activation of impurity and the recovery of defect to carry out a post heat-treatment process at a temperature of 600 to 950° C.
In recent years, an active matrix type liquid display device, which uses a TFT (Thin Film Transistor) provided on a glass substrate as a switching element of pixel, has been partly put into practical use. But it is common that these form the source, drain region of TFT as an ohmic contact, with one conductive type amorphous silicon. Also, the structure of TFT takes an inverse stagger type, and as a structural problem, it has been prone to generate a parasitic capacity.
Accordingly, the usage of TFT which forms the source, drain region by self-alignment has been investigated. For that purpose, it has been required to employ an ion-implantation method or an ion-shower method. These methods, however, have been in need of the post heat-treatment process at a temperature of 600 to 950° C., so as to activate an impurity and recover a defect, as mentioned above, and it has been industrially difficult to use them, considering that a heat resistant temperature of a cheap glass substrate is in the range of 600 to 700° C.
To solve such heat damage problem as is given to a glass substrate, a doping technology using a laser beam irradiation has been known. For an example of them, there is a method that a thin film of impurity is formed on the semiconductor surface into which a doping is to be effected, then by an irradiation of laser beam, the thin film of impurity and the semiconductor surface is molten to dissolve the impurity. There is also another method that an impurity is added and diffused into a semiconductor through a gaseous phase, by an irradiation of laser beam toward the semiconductor surface, in an atmosphere of reactive gas containing an impurity to be doped. Particularly, in case of using a pulse irradiation type excimer laser, it has a feature that the temperature of glass substrate will only become momentarily around 300 even in using a glass substrate, and then the heat damage to the glass substrate can be beside the question.
The above method to conduct a doping by an irradiation of excimer laser beam does not cause heat damage to a glass substrate. It, therefore, can prevent the substrate from being defective by the heat damage. But it poses a problem that a doping efficiency will be lowered, as an energy will not be absorbed in a reactive gas according to the wave length of laser beam. For instance, in the using of the alexandrite laser beam (wave length 745 nm), PH3 gas can not be decomposed directly. Also, in the using of AsH3 and PH3 containing pentavalent impurity, or B2H6 containing trivalent impurity, as a reactive gas for a doping (doping gas), each gas of them varies in its absorbing wave band. So that, in case where the doping of different element has to be effected, by using various kind of reaction gases, there arises a problem that the doping concentration will be disproportioned.
For example, in case a complementary type device composed of P-channel type TFT (hereinafter referred to as PTFT) and N-channel type TFT (hereinafter referred to as NTFT) is formed, or in case CMOS device is formed, it is needed to use separately each doping gas of N-type providing one and P-type providing one. Therefore, this has been a problem that the doping gas is restricted owing to a sort of laser, and the laser suitable for each doping gas should be respectively prepared.